It has been known for 200 years that muscle contraction can be controlled by applying an electrical stimulus to the associate nerves. Practical long-term application of this knowledge was not possible until the recent development of totally implantable miniature electronic circuits that avoid the risk of infection at the sites of percutaneous connecting wires. One example of this modern technology is the cardiac pacemaker.
A metal of choice in electrode manufacturing has traditionally been titanium. On a fresh titanium surface, however, oxygen ions react with the titanium anode to form an oxide layer. Once a finite oxide thickness has been formed on the surface, polarization increases. The oxide film developed on the surface of a titanium electrode is irreversible. It cannot be reduced to the original metal by passing a charge in the reverse direction. Hence, pure titanium metal is a poor choice for electrodes since it forms a semi-conductive oxide on its surface before and during electrical stimulation. Platinum and stainless steel undergo irreversible dissolution during stimulation as well.
Titanium oxidation reactions are several times more likely in an oxidative environment than those of platinum or platinum alloys, but a thousand times less so than those of stainless steel. Due to the expense of platinum metal and the requirement for large amounts of metal in patch-type electrodes, costs may be too high for the routine use of platinum electrodes.
The charge storage capacity, C, is calculated according to the equation C=(k)(∈)(A/d), where k is the dielectric constant of the film, ∈ is the permissivity in vacuum, A is the true surface area of the film, and d is the thickness of the porous material, it can be seen that in order to achieve a large charge storage capacity (C), the porosity of the dielectric may be maximized with a large film surface area. Numerous types of cardiac pacing and defibrillation electrodes have been developed with these factors in mind, utilizing various configurations and materials asserted to promote lower stimulation thresholds and to improve electrical efficiencies. Thus, for implantable electrode applications, it is desirable to minimize the electrical impedance at the electrode-tissue interface by increasing the intrinsic surface area of the electrode or by reducing formation of the capsule of inactive tissue that surrounds and isolates the electrode from living tissue. Schaldach discusses in detail the selection criteria for implantable electrodes. See M. Schaldach, “Fractal Coated Leads: Advanced Surface Technology for Genuine Sensing and Pacing,” Progress in Biomedical Research, 259-272, June 2000.
Microporous electrodes based on sintered titanium, sintered titanium nitride, and microporous carbon or graphite have been used with some degree of success. However, the electrode reactions in aqueous solutions involve significant gas generation similar to the behavior of titanium. Abrading or sandblasting electrode surfaces is a broadly used method to achieve surface area enhancement. For example, French Patent No. 2,235,666 relates to a stainless steel electrode tip that is sanded to increase surface area and reduce the impedance of the electrode.
Other methods have also been used. U.S. Pat. No. 5,318,572 relates to a 90% platinum-10% iridium porous electrode with recess slots in the shape of a cross and at least one, preferably two variably-sized, porous coatings of 20 to 80 micron diameter 90% platinum-10% iridium spheres deposited on the surface of the electrode. On top of this structure, a reactively sputtered coating of titanium nitride was applied. U.S. Pat. No. 4,156,429 describes a means for increasing the reactive surface area by forming a highly porous sintered electrode body consisting of a bundle of fibers, preferably of platinum but alternatively of ELGILOY, titanium, or a platinum-iridium alloy. Conversely, the fibers may be encompassed within a metallic mesh to yield 70% to 97% porosity. U.S. Pat. No. 5,203,348 relates to defibrillation electrodes that can be formed on titanium ribbons or wires with a sputtered outer layer of platinum, or a silver core in a stainless steel tube with a platinum layer formed onto the tube. U.S. Pat. No. 5,230,337 discloses that the coating is preferably made by sputtering to increase the surface area of the electrode.
U.S. Pat. No. 5,178,957 relates to electrodes and a method of making electrodes including pretreatment of the surface by sputter-etching and sputter-depositing a noble metal on the surface. U.S. Pat. No. 5,074,313 relates to a porous electrode with an enhanced reactive surface wherein surface irregularities are introduced to increase surface area by glow discharge or vapor deposition upon sintered wires. U.S. Pat. No. 4,542,752 describes a platinum or titanium substrate coated with a porous sintered titanium alloy that in turn is coated with a porous carbon. The latter was claimed to promote tissue ingrowth and provide low polarization impedance. U.S. Pat. No. 4,784,161 relates to making a porous pacemaker electrode tip using a porous substrate, where the porous substrate is preferably a non-conductive material such as a ceramic or a polymer made porous by laser drilling, sintering, foaming, etc. to result in pores 5 to 300 microns in depth. U.S. Pat. No. 4,603,704 features a hemispherical electrode made of platinum or titanium, coated with a porous layer consisting of a carbide, nitride, or a carbonitride of at least one of the following metals: titanium, hafnium, molybdenum, niobium, vanadium, or tungsten. U.S. Pat. No. 4,281,668 discloses a vitreous carbon or pyrolytic carbon electrode that is superficially activated, e.g., by oxidation, for microporosity. The electrode is then coated with a biocompatible ion-conducting, hydrophobic plastic.
Despite the numerous means of increasing the surface area to reduce polarization losses and after potentials and the use of noble metals and their alloys as electrodes as described above, with varying degrees of success, there remain significant problems pertaining to polarization losses and sensing difficulties. In order to make further improvements to the electrode, stable oxides of some of these noble metals have been employed as a coating.
It is known that certain metals, metallic oxides, and alloys are stable during electrolysis, and that these metals are useful in a variety of electrode applications, such as chlor-alkali electrolysis (see U.S. Pat. No. 5,298,280). Such metals typically include the elements of the platinum group; namely, ruthenium, rhodium, palladium, osmium, iridium, and platinum. These metals are not suitable for construction of the entire electrode, since their cost is prohibitive. Therefore, these metals or their alloys, or as metallic oxides, have been applied as a thin layer over a base member made of Ti, Ta, Nb, Hf, Zr, or W. These metals are much less expensive than platinum group metals and they have properties that render them corrosion resistant. However, as previously mentioned, they lack good surface electroconductivity because of their tendency to form a surface oxide having poor electroconductivity.
U.S. Pat. No. 5,683,443 discloses implantable stimulation electrodes for living tissue stimulation where the titanium electrodes have metal oxides, such as iridium oxide, applied as coatings on an electrode surface, where the surface area has been increased by mechanical shaping, abrasion by sandblasting, or roughening by chemical etching. The patent also discloses surface area enhancement by applying coatings of metal oxides by virtue of the preferred fit which is possible using mixed sized metal oxide molecules in a lattice arrangement. Thus, a single metal oxide produces a mono-lattice with gaps, but a mixed metal oxide with differently sized molecules produces a binary lattice where the gaps of the mono-lattice may be filled by the smaller of the two molecules.
Iridium oxide may be used as a protective coating for metallic electrodes made of platinum, platinum iridium alloy, stainless steel, stainless steel alloys, titanium, titanium alloys, tantalum, or tantalum alloys. U.S. Pat. No. 4,677,989 discloses a metallic electrode that is made of a metal other than iridium that is coated with iridium oxide to reduce corrosion and to increase charge capacity while being thin, thus allowing charge to flow to living tissue from the electrode. Formation of an iridium oxide coating by a solution chemistry deposition process is discussed.
U.S. Pat. No. 5,632,770 discloses an implantable device with a porous surface coating having an active surface that is substantially larger than the geometric shape of the electrode. The enhanced surface area was achieved by a three dimensional fractal-like geometry that increased the surface area by 1000 fold. A coating such as iridium nitride or iridium oxide applied by vacuum technology, particularly vapor deposition, such as reactive cathode sputtering, CVD, PVD, MOCVD, or ion plating is disclosed.
Electroplated or sputtered iridium oxide on a metal surface cracks and delaminates after a short period of electric current pulsing. Thermo-prepared iridium oxide has no such problem. Some commercially available pacemaker leads lasted for eight years of implantation. It is believed that thermo-prepared iridium oxide has better adhesion to the substrate (usually Ti). High temperature will fuse the iridium oxide coating to the substrate. However, since this process requires high temperature, it is not compatible with materials that are high temperature sensitive.
EIC Laboratories of Norwood, Mass. electroplated iridium oxide on a machined surface of a solid disk of platinum and electroplated iridium oxide on a highly polished thin film of platinum; both had low adhesion strength. Some electrodes had delamination of iridium oxide with static exposure at room temperature to 10% saline. Some electrodes developed cracks after a few voltage cycles (i.e., cyclic voltammetry) −0.6V to +0.8V. The plated layer appears dense and smooth. Sputtered iridium or iridium oxide exhibits the same limitations.
U.S. patent application Ser. No. 10/226,976, titled “Platinum Electrode and Method for Manufacturing the Same,” discloses an alternative fractal platinum material and methods of manufacture and is incorporated herein by reference in its entirety. Due to the superior electrical characteristics of platinum as well as its biocompatibility and stability, platinum is a preferred material for electrodes in harsh environments, such as in a human body. However, because electrodeposited platinum, also called “bright platinum”, has a smooth surface when deposited at a slow deposition rate, its surface area is limited by the geometry of the electrode, and therefore, it is not efficient for transferring electrical charge.
Another form of platinum, known as “platinum black,” is widely known. It is deposited at a high rate and demonstrates high porosity, low strength, a rough surface that has lower bulk density, and less reflectivity of visible light than bright platinum. U.S. Pat. No. 4,240,878 describes a method of plating platinum black.
Platinum black may require additives, such as lead, which promote rapid plating. Lead, however, is a neurotoxin and cannot be used in biological systems. Because platinum black is weak, the thickness of the electroplated layer is quite limited. Thick layers of platinum black are inherently weak and readily flake.
Another form of platinum is “platinum gray,” which possesses intermediate properties between those of bright platinum and platinum black. Formed by electrodeposition at an intermediate rate between that utilized for bright platinum and platinum black, it possesses the desirable high surface area that is characteristic of its fractal morphology. It is strong and can be deposited in thick layers on implantable electrodes. However, it suffers from long-term degradation when exposed to living tissue and when subjected to higher charge density stimulation.
It is desired to have the benefit of a high surface area electrode that is comprised of porous platinum and that is coated with an inert, strong coating that vigorously adheres to the substrate and therefore does not flake off during long-term exposure to living tissue.